They could end up like I did. Had the rig and antenna professionally installed. At 3000 miles the motor started acting up and within 3 weeks of purchase parts were all over the dealers floor. Was fixed and next day same problem surfaced. Dealer took it back and has me in another one. Glad I didn't drill thru roof but mounted it inside the hood lip with an "L" bracket.
Will see how the new Equinox works before I install anything in this one.
Is it possible the problem you had with the car might have been caused by common mode interference from a poorly grounded antenna playing havoc with the engine management when you keyed up? You weren't using it when the garage "repaired" the car but it surfaced when you got it back and was using the radio?